1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic system, and more particularly, to an image forming apparatus using two-component developer in which toner and carrier are mixed for developing device which develops an electrostatic image formed on an image bearing member.
2. Description of Related Art
In a conventional image forming apparatus of an electrophotographic system, there is known a technique in which an electrostatic image formed in the image bearing member is developed using a developer by means of a development device, thereby making the image visible as a toner image. For the development device using the two-component developer in which toner and carrier are mixed as the developer, in order to obtain stable developing characteristics, it is important to keep constant a mixing ratio T/D ratio (D=T+C) (indicating toner density of developer, T/D ratio, hereinafter) of toner (T) and carrier (C) of the developer.
The toner in the developer is consumed at the time of development, and the T/D ratio of the developer is changed. Therefore, it is necessary to appropriately detect the T/D ratio of the developer in the development device, supply toner in accordance with the variation, always control the T/D ratio of the developer constant, and to maintain the quality of image.
Thus, several systems such as light reflection detecting system, an inductance system and a patch detection system have been proposed as developer density detection/control apparatuses (ATR).
In these systems, infrared-emitting LED (light-emitting diode) emits infrared light, and the reflection light is received by a light receiving element, e.g., a PD (photodiode). Many image forming apparatuses using the light reflection detection system which detects the T/D ratio of a developer by measuring the amount of reflection light are used (Japanese Patent Application Laid-open No. 2002-091156).
This is based on a principle that toner reflects infrared light carrier and carrier absorbs infrared light, and if the amount of toner is increased, the reflection light amount is increased, and if the amount of toner is reduced, the reflection light amount is also reduced. Therefore, if the supply amount of toner is adjusted such that the reflection light amount becomes a constant value, the T/D ratio of the developer can be maintained at a constant level.
In a full color image forming apparatus, a development device having a plurality of development devices using developers of different colors is used. Among them, a color image forming apparatus for forming a full color image requires development devices of three colors, i.e., yellow, magenta and cyan, or four colors including black.
If the plurality of development devices are respectively provided with a developer density detecting unit, the same number of developer density detecting units as that of the development devices is required. Further, the same number of signal processing circuits which processes a detection signal from the developer density detecting unit and outputs a developer (toner) supply signal as that of the developer density detecting unit is required. Thus, the structure of the apparatus is complicated, and cost is largely increased.
To solve this problem, the apparatus has a moving type development device which can move a specific development device at the time of developing operation to develop, and a light reflection detecting unit detects a developer carried on a developer bearing member of the development device. An image forming apparatus capable of controlling the T/D ratio of each developer using a single developer density control apparatus is proposed (see Japanese Patent Application Laid-open No. 05-313495).
In the developer density control (ATR) of the conventional light reflection detection system, however, the output of the light reflection detecting unit is largely varied by variation of the developer coat amount carried on a developing sleeve which is the developer bearing member of the development device. Thus, there is an adverse possibility that a value is erroneously be detected.
The developer coat amount which is carried on the developing sleeve is varied by variation in fluidity of developer caused by a large amount of image forming operation, variation in surface properties (surface roughness, friction coefficient) of the developing sleeve, and an image ratio of an output image.
In the conventional image forming apparatus, as a material of the developing sleeve, aluminum or stainless steel (SUS) provided at its surface with projections and depressions to enhance bearing and transportation ability of developer is frequently used.
In such a developing sleeve, if the coat amount of developer carried on its surface is reduced, the surface is exposed. In addition, the material such as aluminum and stainless steel constituting the developing sleeve has high reflection coefficient, the reflection light from the developing sleeve surface is increased when the developer density is detected by the light reflection detecting unit. Thus, if the developer density is high, erroneous detection is caused, and control for reducing the supply amount of toner is adversely performed to reduce the developer density by the developer density control.
As a result, the developer coat amount on the developing sleeve is reduced and developing performance is deteriorated, and the developer density is reduced by the developer density control. Therefore, the developing performance is abruptly deteriorated, the image density is reduced and carrier is adhered.